The present invention relates to mobile radio communications, and more particularly, to reliable congestion control in a CDMA-based mobile radio communications system.
In a mobile radio communications system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are usually connected to a switching node, which is typically connected to a gateway that interfaces the mobile radio communications system with other communications systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more nodes to a base stations which serves the called mobile station. The base station pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction; however, paging is not performed.
In a spread spectrum, Code Division Multiple Access (CDMA) mobile communications system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same frequency band. In other words, individual radio xe2x80x9cchannelsxe2x80x9d correspond to and are discriminated on the basis of these codes. Each coded signal overlaps all of the other coded signals as well as noise-related signals in both frequency and time. By correlating a composite signal with one of the distinguishing spreading codes, the corresponding information can be isolated and decoded.
Spread spectrum communications permit mobile station transmissions to be received at two or more xe2x80x9cdiversexe2x80x9d base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a xe2x80x9chandoverxe2x80x9d from one base station to another without any perceptible disturbance in the voice or data communications. This type of handover is typically called diversity handover and may include a xe2x80x9csoftxe2x80x9d handover between two base stations and a xe2x80x9csofterxe2x80x9d diversity handover between two different antenna sectors connected to the same, multi-sectored base station.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user""s communication interferes with the communications of other users. In addition, signals received by a base station from a mobile station that is close to the base station are much stronger than signals received from other mobile stations located at the base station cell boundary. As a result, close-in mobile stations may overshadow and dominate more distant mobile communications, which is why this condition is sometimes referred to as the xe2x80x9cnear-far effect.xe2x80x9d Thus, control of mobile transmit power level is important in order to prevent such near-far effects. Power control is also needed to compensate for changing physical characteristics of a radio channel. Indeed, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. Consequently, large differences may arise in the strength of signals received at the base station from different mobiles.
Ideally, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of the path loss to the base station. By regulating transmit power to the minimum necessary to maintain satisfactory call quality, capacity at the mobile radio communications system can be increased approximately seventy percent as compared with an unregulated system, (assuming that all the calls or connections have the same target signal-to-interference ratio). In addition, mobile stations consume less energy when transmit power levels are maintained at a lowest possible level, thereby reducing battery drain which results in mobile stations lighter in weight and smaller in size.
If the transmission power from a mobile signal is too low, (for whatever reason), the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Erroneous receipt of signals adds to the delay associated with radio access procedures, increases signal processing overhead, and reduces the available radio bandwidth because erroneously received signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system.
A significant problem in CDMA systems with transmitting too much power is the so-called xe2x80x9cparty effect.xe2x80x9d If one mobile transmits at too high of a power level, (a person is talking too loudly at a party), the other mobiles may increase their power levels so that they can be xe2x80x9cheard,xe2x80x9d (over the loud talker), compounding the already serious interference problem. As each mobile increases its transmit power, (becomes a loud talker), the other mobiles react by raising their transmit powers. Soon all mobiles may be transmitting at maximum power with significantly degraded service and diminished capacity. Thus, while transmit power control is important in any mobile radio communications system, it is particularly important to the performance and capacity of a CDMA-based mobile radio communications system.
One parameter affecting the capacity of a CDMA-based system that can be measured by a base station is the total uplink (from mobile station-to-base station) interference level at the base station. The uplink interference includes the sum of all radio beams that reach a receiver in the base station for a specific radio frequency carrier, plus any received noise or interference from other sources. Because of the importance of interference level to the capacity of the CDMA-based radio network, a radio network controller normally receives measurement reports from radio base stations including periodic uplink interference and downlink power measurements. These measurement reports may be used by call admission and congestion control functions of the radio network controller. If the downlink and uplink interference levels are sufficiently low, the admission control function may xe2x80x9cadmitxe2x80x9d a new call request and allocate the appropriate radio resources, assuming other conditions are met, e.g., there are sufficient radio resources currently available. However, if there are insufficient resources or the cell is at capacity or in an overload condition, the admission control function may restrict or reduce the amount of traffic and thereby interference. For example, new mobile connection requests may be rejected, data throughput may be reduced, data packets delayed, handovers to other frequencies/cells forced to occur, connections terminated, etc. Of course, these types of actions should be employed only where necessary; otherwise, the cellular network services and capacity are unnecessarily reduced.
Accordingly, it is an important goal in a cellular radio system to optimize the capacity of a particular cell without overloading that cell. Some type of metric is needed that provides an accurate measurement or other indicator of the current capacity, congestion level, or load in a cell. One possible metric is received signal strength as measured by the base station. Measurement of total received signal strength can be made using some sort of power sensor such as a diode or a resistor. For example, the total received voltage detected across the diode or the total heat generated by the resistor can be used to indicate the total received signal strength.
Unfortunately, a limitation with this measurement-based metric is accuracy. It is very difficult to accurately measure total uplink received power using these types of sensors because the outputs of such sensors change with temperature, aging, component tolerances, etc. Thus, while a desired measurement accuracy of the total uplink received power or interference level may be +/xe2x88x921 dB (or less), the actual measurement accuracy possible with such absolute value measurement techniques may only be +/xe2x88x923-5 dB, when considering economic and product restrictions like manufacturing cost, volume, power consumption, etc.
Such a margin means that the maximum capacity of a cell must account for this uncertainty. To guarantee that the power or interference level does not exceed a particular maximum value in a cell, it is necessary to include a margin that equals the largest possible error. In other words, the maximum capacity for a cell must be designed lower than necessary in order to account for the fact that the power or interference level measurement might well be 5 dB lower than the actual power or interference level. The price for such margins of safety because of uncertain measurement is high. The loss in capacity between a power or interference level measurement uncertainty of +/xe2x88x921 dB and +/xe2x88x923-5 dB is on the order of twenty to forty percent.
Another metric that might overcome the difficulties with accurately measuring the absolute received power or interference level in a cell is a measurement of the variance or standard deviation of received power or interference. This variance metric is useful because it only measuring a relative value, i.e., how fast the measured power is changing. Thus, the absolute measurement accuracy is not as important as with the previous metric. The underlying premise of such a variance metric is that as the loading of a cell increases, so does the variance of the received power. One problem with this approach is that too much time is needed to obtain the necessary statistics to calculate the variance.
As described in the commonly-assigned related application referenced above, the present invention employs a metric that overcomes the problems with the absolute measurement and variance measurement metrics. Rather than measuring the absolute or relative value of a particular radio parameter or condition in a cell, e.g., congestion, overload, power or interference level, etc., the load situation of a cell is determined without the need to measure that load condition by counting transmit power control commands issued in the cell. Based on those issued commands, the condition of the cell may be regulated if desired, e.g., admission and/or congestion control, transmit power control, etc. In one example implementation, the number of increase transmit power commands issued in a cell over a particular time period is determined relative to a total number of transmit power commands, (i.e., both increase and decrease), issued in the cell for that same time period. If the number of increase transmit power commands relative to the total number of transmit power commands exceeds a threshold, an overload condition may be indicated. When an overload is indicated, an action may be taken that reduces the number of increase transmit power commands issued in the cell.
One drawback regulating a cell condition based on observed values of transmit power control commands issued in the cell is that there may be situations in which those commands are not reliable because they are not implemented by the receiving node. For example, if the transmitted commands are corrupted or lost over the radio interface, they will not be implemented. In diversity handover situations, a mobile station follows power control commands of the xe2x80x9cdominantxe2x80x9d base station that is usually satisfied with the lowest transmitted power, i.e., the base station with the lowest path loss. The other diversity base stations typically would like the mobile station to transmit with more power because they are farther away and have a greater path loss. Accordingly, the dominant diversity base station will likely transmit more power-down commands than non-dominant diversity base stations. The mobile station will not follow both a power-up and a power-down command at the same time. One of the commands is ignored and most often, it is the power-up command. Ignored commands should not be counted. Still further, a mobile station may not be able to further increase its power due to the fact that it has reached its maximum power limit. At that point, any power-up command is ignored. However, the ignored commands are counted. In all of these situations, all the transmit power commands would be normally counted as valid if they were issued in the cell, irrespective of whether they have not been implemented. Such an indiscriminate command count value is not reliable. Unreliable count values may result in xe2x80x9cfalse alarmsxe2x80x9d or other undesired network reactions.
One approach to avoid such false alarms or other undesired network reactions, as described in the commonly-assigned, above-referenced application, is to measure the interference level in the cell as a xe2x80x9cdouble checkxe2x80x9d of the transmit power control command count output. However, as already explained, inexpensive interference measurement implementations are inaccurate. In addition, the measurement of uplink interference also contains power from external sources that cannot be filtered out which further decreases the accuracy of that measurement.
The present invention provides an inexpensive and effective way to provide reliable cell congestion control. Transmit power control commands issued in the cell are detected, and the reliability of the issued transmit power commands is determined. A condition in the cell, e.g., congestion level, is controlled based on those issued transmit power commands which are determined to be reliable. Reliably issued transmit power control commands are consistent with a corresponding transmit power control decision made by the receiver of the command, where the decision is not influenced by a maximum or minimum power level. One example of a reliable command includes a command that is actually implemented by the receiver of the command. Those commands that are inconsistent with the mobile""s transmit power control decision, (one example being commands that are not implemented), are deemed unreliable, and therefore are not considered in controlling the condition of the cell. In a preferred example implementation of the present invention, only reliably issued and implemented transmit power control commands are counted, and that count value is used to control the cell condition.
In a diversity handover situation between a mobile terminal and first and second diversity base stations, unreliable transmit power control commands include those issued from one of the diversity base stations which are ignored by the mobile terminal. In one example implementation of the invention in this context, only one of the diversity base stations is selected for each of multiple time periods during that diversity handover communication. Those commands issued by a selected base station may be treated as reliable. Another measure of reliability could be the frequency with which one of the diversity base stations is selected. If that frequency meets a threshold, the transmit power commands issued by that one diversity base station to the mobile terminal are treated as reliable.
In a detailed example implementation of this latter frequency-based approach, the one diversity base station is selected by the mobile station using a site selection diversity transmission (SSDT) indicator. A cell selection comparator generates an output when a SSDT cell identifier broadcast by the mobile station matches an identifier of the current cell. That output is averaged over time. The more selections per time interval, the higher the average. A threshold detector generates an enabling signal if the averaged cell selection comparator output exceeds a threshold. A logic detector passes transmit power control commands to the counter for which the enabling signal is received from the threshold detector.
After determining which issued transmit power control commands are reliable, the number of increase transmit power commands is determined either relative to the total number of reliable transmit power commands issued in the cell or the number of reliable decrease transmit power commands. That number is easily determined using a simple counter. If the counter output exceeds a threshold, action is taken to reduce the number of increase transmit power commands in the cell.
Additional, optional features may be used to advantage along with the reliable power command control procedures. For example, averaging may be employed to reduce reaction to transient fluctuations. Furthermore, it may be desirable to detect the rate of change of a monitored signal in order to vary a threshold at which some network reaction is taken. For fast changes, a threshold value could be decreased to prompt a quicker network reaction to the unstable situation. Still further, it may be desirable to also consider an uplink power or interference value in the cell measured using an inexpensive sensor (despite its inaccuracy) along with the counted reliable transmit power command output as a xe2x80x9cdouble-check.xe2x80x9d
By observing values of reliable transmit power control commands (TPCCs) issued in a cell over a particular time period, the present invention provides an effective, efficient, and inexpensive method to accurately detect and regulate the condition of a cell. Because the TPCC metric is not measured, a margin of error need not be used which may significantly reduce capacity in the cell. The amount of traffic and/or power level in a cell can therefore be regulated to optimize the cell""s capacity without danger of an unstable or undesirable situation, e.g., a xe2x80x9cparty effectxe2x80x9d ramp-up of transmit power/interference. By ensuring that the issued transmit power control commands are reliable and actually implemented, the present invention further enhances the accuracy of the TPCC metric, as well as cell control operations based upon that TPCC metric.